I'd go with superpower. Steel wing is nice for type coverage, but he's not really gonna last against anything with his defenses; even a neutral hit can take him down pretty easily. Superpower is a nice way to break through a few pokemon, and with super luck, it can do a ton of damage at times. Perfect for checking ice pokemon (still one of his weaknesses) rock types (another) and can break through things that try to resist dark like houndoom and Umbreon. The attack and defense drop won't bother you much since he switches out a lot.
